DE 602 12 384 T2 discloses a method for operating a vehicle and/or a method for reversing the driving direction of a moving vehicle, for which a movement of a gear selection device is captured in a position that indicates a new intended driving direction. Such a method for reversing driving direction is often carried out during the operating of a wheel loader. When loading, a wheel loader alternately passes through short routes in the forward and reverse driving directions. Thereby, for example, the vehicle or wheel loader initially moves in the forward driving direction and, upon the presence of a request for a change in driving direction, brakes until it comes to a standstill, and is then operated in a reverse driving direction. Following this, the wheel loader is once again completely braked, and is then operated in the forward driving direction.
In order to avoid a motor of a vehicle being used for the braking of the vehicle when there is a change in driving direction requested by the driver, and the fuel consumption of the motor being increased to an undesired extent, the driving brakes of the vehicle, which are wheel brakes, are actuated depending on the degree of actuation by the driver of a gas pedal of the vehicle, after the gear selection device is transferred into a position equivalent to the new driving direction position. Thereby, it is intended that the extent of the actuation of the driving brakes increases with an increasing actuation of the gas pedal of the vehicle.
A drive train of a vehicle comprises an internal combustion engine in the form of, for example, a diesel engine, an automatic gearbox and/or a main gearbox, and a hydrodynamic torque converter arranged between the main gearbox and the internal combustion engine. The main gearbox is constructed with a forward/reverse gearbox, and a pump for the lift hydraulics of the vehicle is powered by an output shaft of an internal combustion engine. An output shaft of the main gearbox is connected with a differential gearbox of a wheel axle, on which the drive wheels of the vehicle are arranged. Each driving brake is provided in the area of the wheels, in a known manner.
Assuming an operating state of the vehicle, which may be constructed as a wheel loader, with an actual transmission ratio engaged in the main gearbox, is operated with a current vehicle speed in the forward driving direction, and upon a request by the driver for a change in driving direction from a forward to reverse driving direction and the simultaneous selection of a transmission ratio for reverse travel in the main gearbox, the gear selection device is adjusted by the driver into a position equivalent to this. The actuation of the gear selection device by the driver into a new position is recorded by a recording device, which is connected to a control unit.
Based on the movement of the gear selection device and/or the recording of the new position of the gear selection device, the main gearbox of the vehicle is decoupled from the internal combustion engine of the vehicle in the area of the forward/reverse gearbox, by which the motor power is now available in its entirety for the supply of hydraulic functions through the pump and other power consumption devices in the vehicle. Both the motor speed and the braking force provided by the driving brake are increased, while the drive speed is reduced at the same time.
In order to brake the vehicle when change in driving direction is requested, the gas pedal must be pressed by the driver, while at the same time the driving brake is not actuated. This leads to a controlled actuation of driving brakes of the vehicle. For this purpose, the position of the gas pedal is recorded by a second recording device. Thereby, the extent of the actuation of driving brakes is controlled as a function of the position of the gas pedal of the vehicle, whereas the deceleration of the vehicle is adjustable, in a linear or non-linear manner, to a desired extent.
However, it is disadvantageous that, based on the decoupling of the internal combustion engine from the main gearbox, thus from the output of the vehicle, the energy to be expended for the braking of the vehicle is not usable or recoverable for powering the hydraulic consumption devices connected with the internal combustion engine.
For vehicle drive trains without hydrodynamic torque converters constructed with hydrostatic-mechanical power-split gear units, when the driver requests a deceleration of a vehicle, for a simultaneous braking device that is not actuated by the driver, the power flow between a driving unit, such as a diesel engine or the like, and an output is not interrupted, and the drive unit is transferred in an overrun mode, in order to be able to apply a corresponding driving torque on the output. At the same time, the transmission ratio of the gear unit increases during the deceleration process of the vehicle, and the braking action of the drive unit increases, which does not consume any fuel in overrun mode. If, during such an operating state process of a vehicle drive train, an additional consumption device of a vehicle constructed with the vehicle drive train at the same time actuates working hydraulics, the thrust performance of the drive is able to be recovered directly in the vehicle hydraulics, as the power flow in the area of the gearbox between the output and the drive unit is permanently maintained during a deceleration process of the vehicle.
However, it is problematic with this approach that, upon a deceleration process of a vehicle with the starting point of a high speed of the vehicle, the braking action of the drive unit for the implementation of the requested deceleration of the vehicle is able to be realized only if a rotational speed of the drive unit, during the deceleration process of the vehicle, assumes values that adversely affect the functioning of the drive unit under certain circumstances.